1. Field of the Invention
The present invention relates to a magneto-resistive effect (MR) element and particularly to a configuration of a spacer layer.
2. Description of the Related Art
Reproducing heads with high sensitivity and high output are in demand in conjunction with condensing of high recording density in hard disk drives (HDD). As an example of this type of reproducing head, a spin valve head has been developed. A spin valve head includes a nonmagnetic metal layer and a pair of ferromagnetic layers positioned on both sides of the nonmagnetic metal layer in a manner of contacting the nonmagnetic metal layer. The magnetization direction of one side of the ferromagnetic layers is pinned in one direction (hereinafter, this type of layer is referred to as a magnetization pinned layer), and the magnetization direction of the other side freely rotates in response to an external magnetic field (hereinafter, this type of layer is referred to as a magnetization free layer). When an external magnetic field is applied, the relative angle of the spins between the magnetization pinned layer and the magnetization free layer changes so that magneto-resistive change is realized. Typically, the magnetization direction of the magnetization pinned layer is pinned by utilizing the exchange coupling force of an anti-ferromagnetic layer.
On the other hand, in order to realize further condensing of high recording density, a reduction of a read gap (a space between upper and lower shield layers) is required. However, when the read gap is reduced to approximately 20 nm, placing an anti-ferromagnetic layer within the read gap becomes difficult. Therefore, a configuration has been developed in which a pair of magnetization free layers is arranged on both sides of a spacer layer. According to this configuration, the reduction of the read gap is easily realized because no anti-ferromagnetic layer is required.
In order to increase a recording density, not only a reduction of the read gap but also an increase in a magnetoresistance ratio (MR ratio) is required. Particularly, it is desirable to realize a large MR ratio when a resistance-area (RA) is around 0.2 Ωμm2. To achieve that, it is effective to enhance a current density by narrowing a cross section of the spacer layer where a current flows. The specification of the U.S. Patent Application Publication No. 2005/0190510 (Patent document 1) discloses a spacer layer including a plurality of circular-truncated-cone-shaped conductors with an axis in a nearly current direction and an insulator disposed therearound. The specification of the U.S. Patent Application Publication No. 2008/0026253 (Patent document 2) discloses a spacer layer in which micropores having a diameter of 50 nm or less are formed on an MgO layer and metal portions are disposed in the micropores. According to these technologies, it is possible to narrow a cross section where a current substantially flows since a sense current mainly flows in a conductive part. The specification of the U.S. Patent Application Publication No. 2002/0036876 (Patent document 3) discloses an MR element with a cross section where a narrowed current flows. The cross section is narrowed by oxidizing, nitriding or oxynitriding side surfaces of a nonmagnetic metal spacer and magnetic layers.
In order to realize high recording density (for example, 1 Tbpsi or more), it is also required to reduce a space between adjacent tracks of a recording medium. To achieve that, it is required to reduce a track direction width of a spacer layer. A requested width in one example is 35 nm or less. When the track direction width of the spacer layer is small, it is difficult to dispose a conductor, which is disclosed in Patent documents 1 and 2, in the spacer layer in terms of manufacture processes. As described in Patent document 3, with a configuration in which a spacer layer and magnetic layers are oxidized, nitrided or oxynitrided, the reduction of the MR ratio is remarkable because of oxidation of the magnetic layers due to oxidation, nitriding or oxynitriding. Particularly, when the track direction width of the spacer layer is small, it is difficult to control a range of oxidizing, nitriding or oxynitriding properly. Therefore, it is difficult to apply these technologies when the track direction width of the spacer layer is small.
It is an object of the present invention to provide an MR element that can realize a large MR ratio by narrowing a cross section where a current flows even when a track direction width of a spacer layer is small.